1. Field of the Invention
The present invention relates to an acrylic polymer which enables provision of a charge transport material having superior charge transporting capability and also to a charge transport material utilizing the acrylic polymer.
2. Description of Related Art
A variety of charge transport materials have been conventionally used, including inorganic materials such as amorphous Se and Si, and low- and high-molecular organic materials. Among them, organic charge transport materials are widely used for photosensitive electrophotographic materials, because they are less toxic and can be manufactured at low costs.
In recent years, organic charge transport materials have been developed which exhibit superior electrical conductivity and charge transporting capability. Examples of low-molecular organic charge transport materials include Alq3 and starburst amine. Examples of high-molecular organic charge transport materials include polyvinyl carbazole, polyacetylene, polythiophene, polyphenylvinylene and polyfluorene.
Although superior in charge transporting capability, the low-molecular organic charge transport materials suffer from several problems, e.g., in heat resistance and film-forming properties. This has led to recent development of charge transport materials which comprise organic polymers and exhibit good heat resistance and film-forming properties, resulting in the findings that some of them exhibit comparable or even superior physical properties relative to low-molecular charge transport materials.
For example, a 10% isotactic, 39% heterotactic and 51% syndiotactic acrylic polymer obtainable by polymerization of 9-fluorenyl-methacrylate using a free-radical initiator is known (Polymer Journal, Vol. 2, No. 5, pp 555–559 (1971)). This acrylic polymer obtained by polymerization of 9-fluorenyl-methacrylate using a free-radical initiator can be a charge transport material, but its charge transporting capability has been found to be insufficient.
It is understood that in polythiophene or polyfluorene, polymers interact with each other to have an spatial arrangement of π-conjugated groups as in a liquid crystal, thus enabling transportation of charges. However, this stereoregularity changes with temperature and this change has been considered to cause difficulty in transporting charges in a stable fashion.